1. Field of the Invention
This invention relates generally to implantable devices, such as expandable intraluminal prosthesis. More particularly, this invention relates to a stent that delivers a therapeutic substance. Moreover, the present invention relates to a method of delivering a therapeutic substance with a stent.
2. Description of the Background
A variety of surgical procedures and medical devices are currently used to relieve intraluminal constrictions caused by disease or tissue trauma. An example of one such procedure is percutaneous transluminal coronary angioplasty (PTCA). PTCA is a catheter-based technique whereby a balloon catheter is inserted into a blocked or narrowed coronary lumen of the patient. Once the balloon is positioned at the blocked lumen or target site, the balloon is inflated causing the remodeling of the lumen. The catheter is then removed from the target site thereby allowing blood to freely flow through the lumen.
Although PTCA and related procedures aid in alleviating intraluminal constrictions, such constrictions or blockages reoccur in many cases. The cause of these recurring obstructions, termed restenosis, is due to the body's immune system responding to the trauma of the surgical procedure. As a result, the PTCA procedure may need to be repeated to repair the damaged lumen.
Stents or drug therapies, either alone or in combination with the PTCA procedure, are often used to avoid or mitigate the effects of restenosis at the surgical site. In general, stents are small, cylindrical devices whose structure serves to create or maintain an unobstructed opening within a lumen. The stents are typically made of, for example, stainless steel, Nitinol or other materials and are delivered to the target site via a balloon catheter. Although stents are effective in opening the stenotic lumen, the foreign material and structure of the stents themselves may exacerbate the occurrence of restenosis or thrombosis.
Drugs or therapeutic agents that limit migration and/or proliferation of vascular smooth muscle cells are used to significantly reduce the incidence of restenosis and thrombosis. Examples of various therapeutic agents commonly used include heparin, antithrombogenic agents, steroids, ibuprofen, antimicrobials, antibiotics, antiproliferatives, tissue plasma activator inhibitors, monoclonal antibodies, antiinflammatory substances, and antifibrosis agents.
Should the therapeutic agents be applied systemically to the patient, they are absorbed not only by the tissues at the target site, but by all areas of the body. As such, one drawback associated with the systemic application of drugs is that areas of the body not needing treatment are also affected. To provide a more site-specific treatment, stents are frequently used as a means of delivering the drugs exclusively to the target site. The drugs are included or incorporated in a tissue-compatible polymer, such as a silicone, polyurethane, polyester, hydrogel, hyaluronate, and various copolymers and blended mixtures thereof. By positioning the stent at the target site, the drugs can be applied directly to the area of the lumen requiring therapy.
The above-described device, for treatment of restenosis and thrombosis, offers many advantages to potential patients. However, such devices may be deficient in their current drug-delivery capabilities. In particular, restenosis does not necessarily develop at a constant rate. The polymer-coated device may have limited effectiveness because the therapeutic agents are released by passive diffusion, and therefore do not have a release pattern that corresponds to the pathological cascade of restenosis.
In view of the above, it is apparent that there is a need to provide a drug delivery device which can control the release of the therapeutic agents so that conditions such as restenosis, that develop at a variable rate, can be more effectively treated.